The present invention relates to an improved method of driving an ultrasonic motor in which motive force is produced by elastic travelling waves generated by vibration of a piezoelectric body.
An ultrasonic motor, (for example as described in U.S. patent application Ser. No. 706387) consists of a vibration body which is formed of a piezoelectric body and an elastic body, and a rotation body which is disposed in pressure contact with the vibration body. By applying an alternating electric field to the piezoelectric body, elastic travelling waves are generated by vibration of the vibration body, whereby the rotation body is rotated by the effects of frictional force.
The resonance characteristic of the vibration body is almost identical to that of the piezoelectric body alone. As a result, efficient driving operation with a minimum level of field strength can be attained by driving with an alternating electric field whose frequency is close to the resonance frequency of the vibration body. However since the resonance frequency of the vibration body will vary in accordance with operating temperature and motor load, corresponding changes will occur in the relationship between drive frequency (i.e. the frequency of the alternating electric field) and the resonance frequency of the vibration body. With prior art methods of driving such an ultrasonic motor, therefore, changes in temperature and load will result in variations in the operating characteristics of the motor, so that it is not possible to maintain a high efficiency of driving the motor. In addition, since a fixed drive frequency and fixed level of drive voltage are applied to the ultrasonic motor with such prior art drive methods, such changes in the operating characteristics due to temperature or load variations may result in changes in the speed of rotation of the motor, i.e. stable operation cannot be ensured with respect to such temperature and load variations.